Method and apparatus for handling gases in a direct reduction process



March 22, 1960 J. c. AGARWAL ErAL METHOD AND APPARATUS FOR HANDLINGGASES A DIRECT REDUCTION PROCESS Filed may 27, 195s m T W W W d n n .n ua m N A S E R D m 0. J F m Egel .ut I GQQ 2 Q) Q S hst S swab Suki .Q.umnm s. Ent um Il EES@ E 52k "a, Wa/@Sam Allorney limited States PatentMETHOD AND APPARATUS FOR HANDLING GASES A DIRECTREDUCTION PROCESS agdishC.` Agarwal, Verona, Pa., and Fred Kraus and Theodore R. Roszlrows, NewlRochelle, N.Y.; said Agarwal assigner to United States SteelCorporation, a corporation of New Jersey, and said Kraus and Roszkowskiassignors to The Lnmmus Company, New York, N.Y., a corporation ofDelaware Application May 2 7, 195s, serial No. 738,133

1o Claim. (ci. rs-zs) This invention relates to an improved method andapparatus for handling gases in a continuous direct reduction processfor metallic oxides.

Essentially such processes involve continuously contacting metallicoxide lines with a reducing gas at a suitable elevated reactiontemperature. Our invention is particularly applicable to processes inwhich the oxide is iron oxide, the reaction temperature is attained bypreheating the oxide and gas, the reactions take place in lluidized bedreactors,n and the reducing gas is mainly hydrogen, but optionally cancontain carbon monoxide in an amount up to about 25 percent by volume.Nevertheless the invention is not thus limited. As reduction proceeds inthis type of process, the reductants hydrogen and carbon monoxide, ifpresent, are consumed, whil'e their oxidation products water and carbondioxide build up, but substantial quantities of reductant remain in thetinal off-gas from the reactors. Consequently the usual practice is toregenerate this olf-gas by removing oxidation products and then recycleregenerated gas. Water is removed by cooling the o-gas suiciently sothat its vapor condenses, while carbon dioxide, if present, can beremoved in a suitable absorber. Usually a portion of the offgas ispurged from the system to limit build-up of inerts, mainly nitrogen.Fresh reducing gas is added to the regenerated gas to make up for theportions consumed and purged.

Off-gas of course leaves the reactors at a relatively high temperature,and its sensible heat can be conserved by using such olf-gas to heatincoming fresh reducing gas indirectly in a heat exchanger. Fresh gascommonly is produced under high pressure, and the energy which thispressure represents can be conserved by using such fresh gas as motiveuid in a power recovery device, such as a turbine, which assists indriving a compressor for the regenerated olf-gas. VAn earlierapplication of the present co-inventor Agarwal, Serial No. 609,025,filed September 10, 1956, describes and claims several arrangements forthus using this energy.

An object of the present invention is to provide an improved gashandling method and apparatus which utilize more efciently energyrepresented by sensible heat and pressure in the gases, as well asproducing a regenerated gas of enhanced reducing power.

A further object is to provide an improved gas handling method andapparatus in which heat is exchanged between the olf-gas and the freshand regenerated gases in multiple stages, thereby utilizing its sensibleheat more efficiently. v

A further object is to provide an improved gas handling method andapparatus in which olf-gas is cooled to as low as 60 F. to remove watermore completely, a procedure which enhances the reducing power of thegas beyond reasonable expectations. v

In the drawing, the single figure is a simplified schematic owsheet ofour method and apparatus. v

The figure shows a reactor of any conventional 2,92903 Patented Mar. 22,1960 'ice d construction in which ascending gas currents can main tainbeds of finely divided solids in a lluidized state. For simplicity weshow only a single reaction, but it is apparent this reactor can containa plurality of beds in series, either in a single vessel appropriatelypartitioned or in separate vessels, and the reactions therein can takeplace in steps. Metallic oxide fines, preheated to a suitabletemperature (about 1500 to 1800" F. for iron oxide), feed continuouslyto the reactor 10 through a feeder indicated schematically at 12. Areduced product continuously leaves the reactor through a dischargeindicated schematically at 13. Reducing gas, mainly hydrogen, but whichoptionally can contain up to about 25 percent by volume carbon monoxide,is preheated to a suitable temperature (about 1300 to 1600 F. for ironoxide) in a heater 14 of any conventional construction, and isintroduced continuously to the reactor through an inlet line 15. Off-gasleaves the reactor via an outlet line 16, and is at an elevatedtemperature similar to that at which the reduction is carried out (about1200 to 1400 for iron oxide) and a pressure of about 0 to 50 p.s.i.g.

In accordance with our invention (asshown), the outlet line 16 carriesott-gas from reactor 10 through heat exchangers 17, 18 and 19 of anyconventional construction. These heat exchangers utilizesensible heat inthe olf-gas to heat fresh reducing gas Vand regenerated gas in multiplestages, as hereinafter explained. A line 20 lcarlies olf-gas, nowtypically at a temperature of about 300 to 400 F., from heat exchanger19 into a first .scrubber 21 of any conventional construction whereinthe gas is scrubbed to remove dust particles and cooled to a temperatureof about F. to condense out water. Lines 22 carry cooling water into andaway from'thevfir'st scrubber. Gas leaves the first scrubber via a line23` which has an outlet 24 for purging a portion to limit buildup ofinerts. Line 23 carries the gas preferably into a second scrubber 25wherein the gas isrscrubbed with refrigerated water to cool the gas toabout 60 F., and thus condense out more moisture. Lines 26carryvrefrigerated water` into and away from the second scrubber. It ispossible to omit the second scrubber and introduce gas at a temperatureof about 100 F. to the compressor, hereinaftery described. However, weprefer to include this scrubber to cool the gas to about 60 F. tocondense out more moisture, thereby decreasing the power required tooperate the compressor.

A line 27 carries partially regenerated gas from the second scrubber 25into a compressor 28 which compresses this gas to a pressure of about 20to 100 p.s.i.g., and incidentally heats it back to about 200 F. A powerrecovery device 29, such as a turbine of conventional construction,supplies as much as possible of the power needed to operate thecompressor, While a conventional drive 30 supplies the remainder. Thepower recovery device 29 is driven by fresh reducing gas, which isproduced substantially water-free under a relatively high pressure of atleast 20 atmospheres in conventional generating means, not shown. Freshgas enters the system Athrough-a line 31 and passes first through heatexchanger 17, which may be located at any heat exchange position alongline 26 but preferably is the first exchanger, and thence via a line 32to the power recovery device. A line 33 carries the partiallyregenerated and compressed gas from compressor 2S to an after-cooler 34,which partially cools the gas to about 100 F. At this point, vif carbondioxide is to be removed, the gas passes through a conventionalabsorber, not shown. A line 35 carries the gas from the after-cooler 34to a third scrubber 36 into which refrigerated water is introducedthrough lines 26 `to cool the gas again to about 60 F. Because of theincreased pressure, further moisture is condensed out as the gas againis cooled to this temperature (60 E).

On leaving'the third scrubber,

the regenerated gas has a water content of less than 1 percent byvolume, preferably less than about 0.35 percent. A line 37 carries thenow fully regenerated and pressurized gas to exchanger 19,

' which heats it to ya temperature of about 450 to 500 F. by "heatexchange with off-gas from the reactor.

Y In an example of iron oxide reduction, heat exchanger, 17 heats freshreducing gas to a temperature of Vabout 90 to 1000 F., whichtemperature'intentionally isjless than can be attained by exchangeofheat with the offgas in order not to overheat the power recovery device'29@ The power recovery device expands and cools the fresh gas, whichleaves via a line 38a at a pressure of j about 40 to 50 p.s.i.g. and atemperature of about 450 Ato 500 F., values'comparable With those forthe regen- Verated gas leavlng heat exchanger 19 -via a line 38b.

The two gases combine at this stage in a line 38, which carries thecombined gases into heat exchanger 18. The

combined gases have a water content of less than 0.5 percent by volume.Heat Vexchanger 18 heats `the com ybined gases typically to atemperature of about 1000 to 1100 F. A line 39 carries the combinedgases from heat exchanger 18 to the gas heater 14 which heats theVgasesrto their final temperature prior to introduction to reactor 10. Y

" j One way of practicing our invention-,is illustrated by Vthefollowing specific example, in which the metallic oxidefcomesfromriron'ore containing V97 percent FeZOa and the balance siliciousgangue. I is performed in two steps,iirst to an intermediate ,productpredominantly FeO and second to ,a final `product' pre- Reduction ofthis oxide ',dominantly metallic iron. The reducing gas introduced tothe reactor v10 has approximately ,the composition:

Percent by volume 'Hydrogen ,Y 84.50

Water 0.255

Inerts This gas is preheated in the heater 14 to about 17600 F.,

lthe ore is preheated to about 1700 F., and reaction temperatures ofabout 1300" F. are maintained. Y The gas is Y' 4 of about 420 p.s.i.g.and having approximately the folvlowing composition:

Percent by volume Hydrogen 98.5 Water v 'j p j Trace Inerts -..1., v1.5

Heat exchanger l17 rases the temperature -ofzthis gas to about 950 F.The'power recovery deviceo29vlowers the 'temperature to about Y481'" F.and the pressure to about 44V p.s.i.g. Heat exchanger I18 V.heatsthecombined regenerated and `fresh' reducing gases to a temperature ofabout 1015" F.; consequently the Ygas heater 14 is required to'heat thegas only by about an additional 600 F. The power recovery device 29supplies approximately 40 to k50 percent of the energy needed todrive-the compressor 28,Y thus materially decreasing the size of drivemotor 30 and the power requirements therefor. By using the abovearrangement ofrheatY exchangers,Y we realize a Ymaximum of heat recoveryfrom the off-gas and a maximum conversion of heat energy into'useablemechanical energy. c

`Heretofore the usual practice has beenV to cool off-gas only to about100 F. to condense Vout water during the regeneration process, leavingwater content of about 1.5

percent by volume in the'ol-gas. Consequently the under a pressure ofVabout 27.9 p.s.i.g. as it enters theV reactor. On the basis of 100pound'mols lper hour of lFezOaintroducedjto the reactor, about 1045pound mols c per lhour ofj gas are introduced, and the controlling r6-reaches about 72.6 percent of equilibrium. Y v

Off-gas from the reactor is at a temperature of about 1300D F., apressure of 14 p.s.i.g., and has approximately the followingcomposition: Y Y

Percent by volume Hydrogen 57.3 Water 27.5

Inerts As this gas passes through heat exchangers 17, 18 and 19, its`temperature drops successively to about 1115" F., :625 F. and 325 F.,and as it lpasses through the scrubbers 21and Y25, its temperature dropssuccessively to about A10,0." Rand 60 vF. inthe scrubbers the watercontent drops successively to 4.275 percent and l1f.,235per cent. Thecompressor 28 raises the pressnreoof the gas from about 5 p.s.i.g.torment-49.5 p.s.i.g., and itsV temperwater content'of reducing gasentering the reactor (re- Agenerated o-gas plusfresh gas) hascbeen about1.17

percent by volume. In reducing iron oxide to a 95 per- `cent reducedproduct with gas of this water content and a reaction temperature ofabout 71300 F., we have not .been able to reach any more than about 73percent of .equilibrium lin 4thecontrolling reaction:

'If the reducing gas contains 15.2 percent inerts, as in the pvforegoing example, o'l-gasfrom thistreaction has a water content of18.6 percent `by volume at a 73 percent approach to equilibrium. Waterinthe reducing gas thus causes a loss of 6.3 percent vin the reducingycapacity of the gas. By cooling theoi-gas to 60 F. afterrcompressingit, we lower .the water content of gas introduced to the reactor(regeneratedoi-gas plus ,fresh gas) to Vabout0.255 percent by volume inthe example. The loss in reducing capacity caused by water in thereducing gas iscnt to about 1.37 percent, assuming the same approach toequilibrium. 1 ,t

' YFrom the foregoing description, .it is seen that our inventionincreases the etliciencyv of a direct reduction process in severalimportant respects, namelyrin utilization of sensible heat in oi-gases,in utilization of inherent high pressure of fresh reducing gases, and incutc reducing gas consisting Vmainly of hydrogenfcontactsiiron 1oxidetnes at :a temperature of about 120,0 to 117400 P., ,l therebyreducingthe oxide and'oxidizing 1a portion .0f

. the hydrogen to water vapor, off-gasfrom the reducing -ature backtoabou-t F.' at the exit ofthe aftercooler 34. As the gas passes throughthe' third scrubber 36, its temperature ,again rdrops to about 60 F. andits water contentl to about 0.353 percent. Heat exchanger :19 raises thetemperatureto about 480 After thus c regenerating and purging, about 735mols of gas remain.

310 mols of fresh reducing gas Yareintroduced to the system at atemperaturesof about 100 F. and a pressure step is regenerated lbyvcooling 'it-to condense Vout water` and compressedfresh reducing Ygasis combined with the o regenerated gas, and the combined .gasesagaincontact roxide fines, a method of handling the. gases A.to conserveenergy represented by sensible vheat in the off-gas and pressurein thefresh gas` comprising exchanging heat between the oft-gas and the freshgas, between @the oigas and the combined gases, Yand between the Yo-ga'sand the compressed regenerated gas before thel off-gas is cooled tocondense out water, and obtaining energy for compressing the regeneratedgas from pressure in the fresh gas and thereby expanding and cooling thefresh gas, the combining of regenerated and fresh gases taking placefollowing the exchange of heat between the off-gas and the regeneratedgas and following the expansion and cooling of the fresh gas.

2. In a continuous direct reduction process, wherein reducing gasconsisting mainly of hydrogen contacts iron oxide fines at a temperatureof about 1200 to 1400 F., thereby reducing the oxide and oxidizing aportion of the hydrogen to water vapor, off-gas from the reducing stepis regenerated by cooling it to. condense out water and compressed and aportion purged to limit build-up of inerts, fresh reducing gas iscombined with the regenerated gas, and the combined gases again contactoxide fines, a method of handling the gases to conserve energyrepresented by sensible heat in the olf-gas and pressure in the freshgas comprising exchanging heat between the olf-gas and the fresh gas,between the olf-gas and the combined gases, and between the off-gas andthe compressed regenerated gas, and obtaining energy from pressure inthe fresh gas after exchange of heat with the offgas for compressing theregenerated gas and thereby expanding and cooling the fresh gas, thecombining of regenerated and fresh gases taking place following theexchange of heat between the olf-gas and the regenerated gas andfollowing expansion and cooling of the fresh gas.

3. A method as defined in claim 2 in which a portion of the cooling ofthe off-gas takes place following the exchanges of heat but beforecompression of this gas, and the remainder of the cooling of the o-gastakes place following compression.

4. In a continuous direct reduction process, wherein reducing gasconsisting mainly of hydrogen contacts iron oxide fines at a temperatureof about 1200 to 1400 F., thereby reducing the oxide and oxidizing aportion of the hydrogen to water vapor, off-gas from the reducing stepis regenerated by cooling it to condense out water and compressed and aportion purged to limit build-up of inerts, fresh reducing gas iscombined with the regenerated gas, and the combined gases again contactoxide fines, a method of handling the gases to conserve energyrepresented by sensible heat in the olf-gas and pressure in the freshgas comprising exchanging heat between the oifgas and the fresh gas,between the o-gas and the combined gases, and between the oif-gas andthe compressed regenerated gas, thereafter cooling the off-gas,compressing the cooled olf-gas and incidentally heating it, cooling thecompressed gas again thereby completing its regeneration, and obtainingenergy for compressing the cooled olf-gas from pressure in the fresh gasand thereby expanding and cooling the fresh gas, the combining ofregenerated and fresh gases taking place following the exchange of heatbetween the off-gas and the regenerated gas and following expansion andcooling of the fresh gas.

5. In a continuous direct reduction process, wherein reducing gasconsisting mainly of hydrogen contacts iron oxide fines at a temperatureof about 1200" to 1400 F., thereby reducing the iron oxide and oxidizinga portion of the hydrogen to water vapor, o-gas from the reducing stepis regenerated by cooling it to condense out water and compressed and aportion purged to limit build-up of inerts, fresh reducing gas iscombined with the regenerated gas, and the combined gases again contactiron oxide nes, a method of handling the gases to conserve energyrepresented by sensible heat in the off-gas and pressure in the freshgas comprising exchanging heat between the olf-gas and the fresh gas,between the off-gas and the combined gases, and between the off-gas andthe compressed regenerated gas, thereafter cooling the off-gas to about60 F., compressing the cooled o-gas and incidentally heating it to about200 F. by energy in part obtained from pressure in the fresh gas afterexchange of heat with Y(i the off-gas, thereby expanding and cooling thefresh gas, and cooling the compressed gas again to about F. therebycompleting its regeneration, the combining of regenerated and freshgases taking place following exchange of heat between the olf-gas andthe regenerated gas and following expansion and cooling of the freshgas.

6. In a continuous direct reduction process, wherein reducing gasconsisting mainly of hydrogen contacts iron oxide lines at an elevatedtemperature, thereby reducing the iron oxide and oxidizing a portion ofthe hydrogen to water vapor, olf-gas leaves the reducing step at atemperature of about 1200 to 1400 F. and a pressure of 0 to 50 p.s.i.g.and is regenerated by cooling it to condense out water and compressedand a portionpurged to limit build-up of inerts, fresh reducing gasproduced at a pressure of at least 20 atmospheres is combined with theregenerated gas, and the combined gases heated to a temperature of about1300" to 1600o F. and again contact iron oxide iines, a method ofhandling the gases to conserve energy represented by sensible heat inthe oE-gas and pressure in the fresh gas comprising exchanging heatbetween the off-gas and the fresh gas, between the off-gas and thecombined gases, and between the off-gas and the compressed regeneratedgas, thereafter cooling the off-gas to about 60 F., compressing thecooled off-gas and incidentally reheating it to about 200 F., coolingthe compressed gas to about 60 F. thereby completing its regeneration,and obtaining energy from pressure in the fresh gas after exchange ofheat with the o-gas for compressing the regenerated gas and therebyexpanding and cooling the fresh gas to a pressure and temperaturecomparable with the regenerated gas following its heat exchange with theol`f-gas, the combining of regenerated and fresh gases taking placefollowing the exchange of heat between the off-gas and the regeneratedgas and following expansion and cooling of the fresh gas.

7. In a continuous direct reduction apparatus which includes a reactor,means for feeding preheated metallic oxide to said reactor and fordischarging reduced product therefrom, a gas heater, an inlet forintroducing preheated reducing gas from said heater into said reactor,an outlet for removing olf-gas from said reactor, and means forsupplying fresh reducing gas, the combination therewith of a gashandling apparatus comprising at least three heat exchangers in seriesconnected to receive offgas from said outlet, cooling means connected toreceive oftlgas from the last heat exchanger of said series forcondensing out water therefrom, a compressor connected to receive gasfrom said cooling means for compressing and incidentally heating thecooled off gas, cooling means connected to receive gas from saidcompressor for condensing out more water therefrom and thus completingits regeneration, one heat exchanger of said series being connected toreceive gas from said second named cooling means for reheating theregenerated gas from sensible heat in the off-gas, another heatexchanger of said Y series being connected to receive gas from saidfresh gas supplying means for heating fresh gas from sensible heat inthe o-gas, and expanding and cooling means connected to receive freshgas from said last mentioned heat exchanger, a third heat exchanger ofsaid series being connected to receive combined regenerated gas fromsaid one heat exchanger and fresh gas from said expanding and coolingmeans for heating the combined gases from sensible heat in the ofi-gas,said heater being connected to receive combined gas from said third heatexchanger.

8. A combination as defined in claim 7 in which said expanding andcooling means includes a turbine drivingly connected to said compressor.

9. In a continuous direct reduction apparatus which includes a reactor,means for feeding preheated metallic oxide to said reactor and fordischarging reduced product therefrom, a gas heater, an inlet forintroducing preheated reducing gas from said heater into said reactor,an outlet for removing off-gas from said reactor, gas

v 7K W. Y cooling and compressing meansfm regeneratin'g'the ogas, andmeans for supplying .fresh reducinggasandcombining it with regenerated.ntf-gas, the combination there-1 with of a Ygas vhandling apparatuscomprising first, second and third heatexchangers in seriesconnected-toffeecive oi-gasfrom said outlet, said Ycooling meansincluding a scrubber connected .to'receive off-gas from said third heatexchanger, said v:compressing means being connected to receive gas .fromsaid-scrubber, said .coolingmeans Yincluding .anotherscrubherxconnectedrto receive gas from saidY compressingmeans, saidAthird'Y heat .exchanger being connected to receive Ygas fromrsaidsecond named-scrub,.- ber for reheating this gas fromcsensible heat infthe'of# gas, .said first heat exchanger beingconnected to receive gasfromesad .freshvgas supplyingrmeansfor heating fresh gas .from sensibleheat in .the olf-gas, and a .turbine vconnected to receive fresh gasfrom said rst heat exchanger and expand and cool the fresh gas, saidsecond heat ex changer being Aconnected to receive-,combined regeneratedsaid turbine vfor heating the combined gases from sensible heat in theoff-gas, .said heater fbe'ing connectedqto areb ceive'combined gsfromsaid :secondhea't exchanger.

Y 10. A.combination` as defined -in claim' 9i in which said turbine is:drivingly connectedft'orsaid compressing` means. Y V- z(-ReferencesCited'in'the Yfile of thispatent 'Y v UNITEDY STATESPATENTSV 2,107,549 Schmalfeldt Febf, 193.8 -2,142,100 Avery,-.. Jan.3,'1939 2,547,685 Brassert etal.' V-..:Apr. 3.11951 2,584,570 Messingeret al Feb. A12, .1952 2,671,765 McGrath et al.A Mal-.9,1954 .2,752,234VShipley Iunel, 1-956

1. IN A CONTINUOUS DIRECT REDUCTION PROCESS, WHEREIN REDUCING A GASCONSISTING MAINLY OF HYDROGEN CONTACTS IRON OXIDE FINES AT A TEMPERATUREOF ABOUT 1200* TO 1400* F., THEREBY REDUCING THE OXIDE AND OXIDIZING APORTION OF THE HYDROGEN OF WATER VAPOR, OFF-GAS FROM THE REDUCING STEPIS REGENERATED BY COOLING IT TO CONDENSE OUT WATER AND COMPRESSED, FRESHREDUCING GAS IS COMBINED WITH THE REGENERATED GAS, AND THE COMBINEDGASES AGAIN CONTACT OXIDE FINES, A METHOD OF HANDLING THE GASES TOCONSERVE ENERGY REPRESENTED BY SENSIBLE HEAT IN THE OFF-GAS AND PRESSUREIN THE FRESH GAS COMPRISING EXCHANGING HEAT BETWEEN THE OFF-GAS AND THEFRESH GAS, BETWEEN THE OFF-GAS AND THE COMBINED GASES, AND BETWEEN THEOFF-GAS AND THE COMPRESSED REGENERATED GAS BEFORE THE OFF-GAS IS COOLEDTO CONDENSE OUT WATER, AND OBTAINING ENERGY FOR COMPRESSING THEREGENERATED GAS FROM PRESSURE IN THE FRESH GAS AND THEREBY EXPANDING ANDCOOLING THE FRESH GAS, THE COMBINING OF REGENERATED AND FRESH GASESTAKING PLACE FOLLOWING THE EXCHANGE OF HEAT BETWEEN THE OFF-GAS AND THEREGENERATED GAS AND FOLLOWING THE EXPANSION AND COOLING OF THE FRESHGAS.